In order to manufacture a desired semiconductor device, various processes such as a film formation, an etching and the like are repeatedly performed on a semiconductor wafer. Recently, in order to meet demands for high-speed semiconductor device, miniaturization of a wiring pattern and high level of integration, it is required to realize low resistance (high conductivity) of a wiring and high electromigration resistance.
Accordingly, copper (Cu) having a higher electromigration resistance and a higher conductivity (lower resistance) than aluminum (Al) and tungsten (W) used as a wiring material.
As for the Cu wiring structure forming method, there is proposed a method including: forming a barrier film formed of tantalum metal (Ta), titanium (Ti), tantalum nitride (TaN), titanium nitride (TiN) or the like on an entire interlayer insulating film having a trench and/or a hole by a plasma sputtering as an example of a physical vapor deposition (PVD); forming a Cu seed film on the barrier film by the plasma sputtering; filling the trench and/or the hole by performing a Cu plating; and removing a residual Cu thin film or a residual barrier film remaining on the wafer surface by a chemical mechanical polishing (CMP). Then, a cap layer made of a dielectric material such as silicon carbonitride (SiCN), silicon nitride (SiN) or the like is formed on the Cu wiring and thereafter an upper interlayer insulating film is formed (e.g., Japanese Patent Application Publication No. 2006-148075).
However, as a design rule for scaling-down of the aforementioned semiconductor device progresses and a current density is thus increased, the electromigration resistance is not sufficient even when using Cu as the wiring material. Particularly, the electromigration deterioration in the Cu wiring is easily caused by insufficient adhesion between the Cu wiring and a dielectric cap provided thereon.
In view of the above, as for a new technique for improving reliability of a wiring by improving electromigration resistance, there is proposed a wiring forming method using a Cu alloy (Cu—Al, Cu—Mn, Cu—Mg, Cu—Ag, Cu—Sn, Cu—Pb, Cu—Zn, Cu—Pt, Cu—Au, Cu—Ni, Cu—Co or the like) for a seed layer instead of a Cu seed film (e.g., Nogami et. al. IEDM2010 pp 764-767). The alloy component is segregated between the Cu wiring and a dielectric cap (SiCN cap) formed thereon, thereby improving adhesivity therebetween. Accordingly, the electromigration resistance can be improved. The electromigration resistance can also be improved by the segregation of the alloy component of the Cu alloy at grain boundaries of Cu.
In addition, there is proposed a technique for improving electromigration resistance by improving adhesivity between Cu and a dielectric cap by selectively forming a metal cap on a surface of a Cu wiring (e.g., Japanese Patent Application Publication No. 2011-023456 (JP2011-023456A), U.S. Pat. No. 7,799,681 (U.S. Pat. No. 7,799,681B), Japanese Patent Application Publication No. 2012-504347 (JP2012-504347A)).
Meanwhile, in the Cu wiring structure, a Low-k film having a low dielectric constant of about 2.5 is used as a lower interlayer insulating film in order to suppress a resistance-capacitance (RC) delay which is a product of a resistance R of the wiring (line resistance) and a capacitance C between the wirings (line-to-line capacitance). However, in the technique described in Nogami et. al. IEDM2010 pp 764-767, it is difficult to reduce the RC delay because SiCN or the like forming the dielectric cap has a high dielectric constant of about 5 and, thus, an effective dielectric constant of the wiring structure including the cap layer is increased.
In the case of forming the metal cap on the Cu wiring as described in JP2011-023456A, U.S. Pat. No. 7,799,681B, and JP2012-504347A, the metal cap needs to be selectively formed on the Cu wiring in order to reduce a leakage current between wirings. For this reason, the number of processes is increased to ensure the selectivity, so that costs are increased. The metal cap formed by such a technique has poor oxygen barrier characteristics and, thus, moisture in the upper interlayer insulating film or oxygen·moisture in the atmosphere reaches the Cu wiring. Hence, the Cu wiring is oxidized, which causes the electromigration deterioration. Therefore, in order to ensure the barrier characteristics, the dielectric cap is required, and therefore the effective dielectric constant of the wiring structure cannot be reduced.